This invention relates to a control system for a direct current motor energized from a storage battery and more particularly to a control system utilizing a thyristor chopper suitable for controlling a motor for driving an electric motor car.
A control system for controlling a direct current motor by means of a thyristor chopper is more advantageous than a conventional resistance control system in that there is no heat generation at the resistor and that it provides continuous control without using any contact. For this reason, in recent years, the field of application of this type of control system has been widened greatly and application thereof to electric motor cars has been increased. When applied to the control of an electric motor for driving an electric motor car the control system is required to provide a powering control in which the powering operation of the motor is controlled by the current chopping action of the thyristor as well as a regeneration control in which the motor is decelerated by applying thereto an electric braking by regenerating power to the source of supply from the motor. Since these modes of control are performed frequently and repeatedly it is necessary to rapidly and smoothly effect the switching of the circuit connections without using any mechanical contact. The current flowing through a direct current motor controlled by a thyristor chopper is essentially a pulsating current. However, when the frequency of the pulsating current increases, the iron loss and the eddy current loss of the motor increase which results in the decrease in the efficiency and sparking of the brushes caused by an improper commutation. This decreases the life of the brushes associated with the commutator of the motor. In order to decrease pulsation, it is necessary to include a DC smoothing reactor of large capacity in series with the motor. In an electric motor car, miniaturization and decrease in the weight of the machines and apparatus mounted thereon are essential. Moreover, such machines and apparatus are required to be highly efficient for the purpose of minimizing the power consumption of the battery. Safeness and low cost are also desired. Any of the prior art control systems could not satisfy these various requirements. For example, if one attempts to switch the powering-regeneration connections without using any contact or to increase the efficiency by decreasing the pulsation, these attempts rather complicate the circuits and increase the weight.